Process For Applying Lubricants Containing Metal Nanoparticles on Interacting Parts

ABSTRACT

A process for applying a low coefficient of friction coating to interacting parts of a machine. The low coefficient coating is comprised of nanoparticles of a metal melting below about 400° C., preferably bismuth. A dispersion of the nanoparticles in a lubricant oil is introduced into the oil reservoir of the machine and the machine is operated at designed conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of Application 14/705,934filed May 6, 2015 which was based on Provisional Application 61/989,480filed May 6, 2014.

FIELD OF THE INVENTION

This invention relates to a process for applying a low coefficient offriction coating to interacting parts of a machine. The low coefficientcoating is comprised of nanoparticles of a metal melting below about400° C., preferably bismuth. A dispersion of the nanoparticles in alubricant oil is introduced into the oil reservoir of the machine andthe machine is operated at designed conditions.

BACKGROUND OF THE INVENTION

Friction between surfaces of interacting parts of machinery,particularly machinery operating at elevated temperatures, is a majorcause of power consumption and wear. The reduction of friction is a goalfor improving fuel efficiency and for lowering power consumption andwear. For example, friction resulting from interacting surfaces inautomobile and other lubricated machines accounts for about one third ofthe total fuel consumed. Also, for wind turbines, up to one quarter ofoperating and maintenance costs are due to premature replacement of wornmoving parts. One approach for reducing friction resulting frominteracting surfaces of machinery is the use of low coefficient offriction coatings on interacting surfaces. The application of lowcoefficient of friction coatings on interacting machine parts duringmanufacturing of the machine is usually not very successful.Conventional coatings used to provide low coefficients of frictiontypically have a micron size grain structure as opposed to a nano-sizegrain structure. One such conventional coating is a diamond coating thatis expensive to implement into conventional manufacturing processes.Such coating processes are typically limited in the size of the partsthey can coat. In addition, conventional coating processes do typicallynot result in coatings that are capable of preserving the designedclearances between interacting surfaces.

Therefore, there is a need in the art for coatings that: provides a lowcoefficient of friction between interacting surfaces; will preservedesigned clearances between interacting surfaces; will have superiorwear properties, are cost effective to apply.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a process forapplying a low coefficient of friction coating on interaction parts of amachine designed for being lubricated with use of a lubricating oil andhaving a reservoir for storing and providing lubricating oil, whichprocess comprises: i) forming a dispersion of about 0.001 wt. % to about2 wt. % of nanoparticles of one or more metals having a melting pointless than about 400° C. with a lubricating oil; ii) introducing saiddispersion into the reservoir of said machine; and iii) operating themachine under operating conditions for an effective amount of time toform a nano-coating on at least a fraction of the interacting parts ofthe machine.

In a preferred embodiment, the metal is selected from the groupconsisting of bismuth, cadmium, tin, indium, and lead.

In another preferred embodiment the particle size of the metalnanoparticles is from about 2 nm to about 200 nm.

In another preferred embodiment, the interacting parts are manufacturedfrom a material selected from the group consisting of metals, ceramics,and polymeric materials.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 hereof a simplified process flow diagram showing one preferredembodiment for the practice the present invention.

FIG. 2 hereof is a scanning electron photomicrograph of a metal surfacecoated with a bismuth nanoparticle coating of the present invention.This photomicrograph shows that the nano-coating resembles a“cobblestone” structure where the larger, unmelted nanoparticles aredispersed in the smaller, melted nanoparticle, which act as a binderboth to the interacting surfaces and for the larger, solid nanoparticlesof the nano-coating.

FIG. 3 shows time vs coefficient of friction traces obtained by example1 hereof wherein 0.12 wt % of bismuth nanoparticles in lubricating oilAeroshell 555 and Aeroshell 555 alone were separately tested in a Falexmulti-specimen tester with an 88 lb load and a rotation speed of 600RPM. The mean particle size of the nanoparticles was about 60 nm.Coefficient of friction measurements were taken over a period of about60 minutes.

FIG. 4 hereof shows time versus coefficient of friction traces, alsofrom example 1 hereof, with an initial 0.12 wt % bismuth nanoparticle inoil dispersion treatment, then after replacement of thenanoparticle/Aeroshell dispersion with fresh Aeroshell 555 at 75° C.

FIG. 5 hereof shows time versus coefficient of friction traces forexample 2 hereof showing the comparison of pure Aeroshell 555 run in theFalex-multi-specimen tester compared with and 0.06 wt % bismuthnanoparticle dispersion at 90° C. wherein the mean size of thenanoparticles was about 50 nm.

FIG. 6 hereof shows time versus coefficient of friction traces forexample 3 hereof for 25 nm bismuth nanoparticle dispersion in Aeroshell555 dispersion at about 0.05 wt % vs. pure Aeroshell 555 lubricatingoil.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for providing low coefficientof friction coatings having a thickness and grain structure in thenanometer size range and that are capable of preserving the designedclearances between interacting surfaces in machinery requiringlubrication. The term “machinery” or “machine” includes any type ofmechanical power generating device that contains a reservoir for storingand providing a lubricant for interacting parts of the power source.Non-limiting examples of power sources suitable for uses herein includejet, diesel, and gasoline engines, motors, turbines,

The present invention is based on a dispersion of nanoparticles of a lowmelting metal in a lubricant. That is, a metal having a melting pointless than about 400° C. Such metals include bismuth, cadmium, tin,indium, and lead all of which melt below about 400° C. Bismuth ispreferred, and as such, this application will be written primarily interms of bismuth, but it will be understood that any of the otheraforementioned low-melting metals can be used in place of bismuth. Thebismuth nanoparticle oil dispersions of the present invention can beinitially introduced into the oil reservoir of a machine, such as anengine crankcase, gearbox, or a transmission. The machinery is thenoperated under normal operating conditions, preferably under startupconditions, for an effective amount of time to reach the activationtemperature of the nanoparticle dispersion. By effective amount of timewe mean for at least that amount of time wherein at least an effectivepercent of the interacting surfaces are at least partially coated withthe bismuth nanoparticle coating of the present invention. By at leastan effective percent of interacting surfaces coated we mean that atleast that amount of coating is applied that will result in at a least25%, preferably at least a 40%, and more preferably at least a 60%decrease in coefficient of friction compared to the same lubricant butwithout a dispersion of bismuth nanoparticles. It is most preferred thatsubstantially 100% of the interacting surface be coated. That is atleast 98% of the surface be coated.

Another preferred process of applying a low-melting metal nanoparticlecoating onto parts manufactured for use in a machine is to coat theparts before assembly of the machine. It is within the scope of thisinvention that either the entire part can be coated or just theinteracting surface(s) of the part. That is, the targeted surface can beeither the interacting surface of the part of the entire part. The partcan be of a material selected from metal, ceramic, or polymeric. Thetargeted surface to be coated is preferably immersed in a dispersioncomprised of nanoparticles of one or more low-melting metals dispersedin a lubricating oil, preferably a lubricating oil suitable for use inthe machine for which the part was intended. The targeted surface issubmerged in the nanoparticle oil dispersion for an effective amount oftime, which will typically be about one hour or less. By targetedsurface we mean the section(s) of a part that is designed to interactwith one or more surfaces of another part designed to be assembled in amachine. It is preferred that the nanoparticles adhere to at least 25%,more preferably at least 40%, and most preferably from about 50 to 100%of the targeted surface of the part being coated. This allows thelow-melting metal nanoparticles to adhere to the surface of the part bycolloidal or other adhesion mechanism.

The adhesion mechanism plays an important role in the nanoparticlesadhering to, and forming, a coating on the targeted surface. For machineparts that will undergo such movement as sliding, rolling or other typesof surface interactions, the adhesion mechanism without the coatingbeing subjected to an activation temperature, will typically not bestrong enough to keep the nanoparticles from being removed/scrapped fromthe surface during operating conditions of the machine. This unintendedremoval of nanoparticles will substantially reduce the benefits to begained by lowering the coefficient of friction. Thus, it is preferredthat the temperature of the part, or the temperature of thenanoparticle-containing oil dispersion, or both, be at an effectivetemperature during or after the adhesion step. The term “effectivetemperature”, which is sometimes referred to herein as the “activationtemperature” is at least that temperature at which the low-meltingnanoparticles begin to sinter and become strongly attached to thetargeted surface. Since this activation temperature will typicallyexceed the sintering temperature of the low-melting metal nanoparticles,the nanoparticles will start to bond to each other and to the targetedsurfaces they come into contact with. Sintering is the ability ofparticles to form solid coatings and bodies by the diffusion of atomsacross the boundaries of the particles in contact with each other athigh enough temperatures. The upper end of the effective temperaturewill be that temperature wherein the integrity of the nanoparticles, orthe coating, begins to fail. If the nanoparticle dispersion of thepresent invention is introduced into the oil reservoir of an engine ormachine, the activation temperature of the nanoparticle will be reachedduring normal operation of the engine or machine.

After the targeted surface of the part is coated, the temperature islowered to below the sintering or melting point of the low-melting metalof the metal nanoparticle. That is, where the low-melting metal of thenanoparticles liquefies. It is more economical to sinter compacted highmelt point metal/ceramic particles to solid bodies than to use a moreconventional melt and cast procedure

As the particle size of the nanoparticles decreases, the sinteringtemperature also decreases. In the nanoparticle size range for bismuth,the sintering temperature drops below the melting point of 274° C. Inaddition, many metals are soluble in molten bismuth. The bismuth atomsfrom the nanoparticles in contact with the targeted surface are able todiffuse into the metal surface, and dissolve and alloy with the targetedmetal surface. At this stage, the bismuth nanoparticles aresubstantially permanently attached to the surface. The cooling step ispreferred to allow the coating structure to fully form. Attachment toceramic or polymer surfaces is also within the scope of this inventiondue to the highly reactive bismuth atoms from the nanoparticlesdiffusing and forming bonds that attach the nanoparticles to suchsurfaces. The utilization of this technique can be varied in terms ofremoval or retention of the parts in the oil dispersion while heating tothe sintering/activation temperature and is related to the cost ofheating the dispersion/parts, the size of the part, and ability of thedispersion to be used for more than one immersion of the parts in abatch. The parts can also be placed in a heated oil dispersion at theactivation temperature, but care must be taken to prevent thenanoparticles from interacting more with each other than with thetargeted surface and forming larger particles that will not attach tothe targeted surface.

Any suitable lubricant can be used in the practice of present invention.Preferred lubricants are low volatility lubricating oils. Typicallubricating oils are by necessity low volatility to withstand highoperating temperatures. Such oils are prepared from a variety of naturaland synthetic base stocks admixed with various additive packages andsolvents depending upon their intended application. Modern base stocksfor automobile engines typically include mineral oils, polyalphaolefins(PAOs), gas-to-liquid (GTL), silicone oils, phosphate esters, diesters,polyol esters, and the like. Preferred low volatility oils are thosethat will typically be used as the lubricant for the machinery to betreated.

Oils of lubricating viscosity useful in the practice of the presentinvention can be selected from natural lubricating oils, syntheticlubricating oils, mixtures thereof, as well as greases. Natural oilsinclude animal oils and vegetable oils (e.g., castor oil, lard oil);liquid petroleum oils and hydro-refined, solvent-treated or acid-treatedmineral oils or the paraffinic naphthenic and mixedparaffinic-naphthenic types. Oils of lubricating viscosity derived fromcoal or shale also serve as useful base oils. Synthetic lubricating oilsinclude hydrocarbon oils and halo-substituted hydrocarbon oils such aspolymerized and interpolymerized olefins, alkylbenzenes; polyphenyls;and alkylated diphenyl ethers and alkylated diphenyl sulfides andderivative, analogs and homologs thereof. Alkylene oxide polymers, andinterpolymers and derivatives thereof where the terminal hydroxyl groupshave been modified by esterification, etherification, etc., constituteanother class of known synthetic lubricating oil. Another suitable classof synthetic lubricating oils suitable for practice of the presentinvention comprises the esters of dicarboxylic acids with a variety ofalcohols (e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol,2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoether,propylene glycol).

Further, the oil used in the practice of the present invention maycomprise a Group I, Group II, Group III, Group IV or Group V oil orblends of the aforementioned oils. The oil may also comprise a blend ofone or more Group I oils and one or more of Group II, Group III, GroupIV or Group V oil. Definitions for the oils as used herein are the sameas those found in the American Petroleum Institute (API) publication“Engine Oil Licensing and Certification System”, Industry ServicesDepartment, Fourteenth Edition, December 1996, Addendum 1, December1998.

As was previously mentioned, the lubricant used in the practice of thepresent invention can also be a grease. Greases are typically comprisedof oil and/or other fluid lubricant that is mixed with a thickener,typically a soap to form a solid or semisolid. Greases are a type ofshear-thinning or pseudo-plastic fluid, which means that its viscosityis reduced under shear. After sufficient force to shear the grease hasbeen applied, the viscosity drops and approaches that of the baselubricant, such as a mineral oil. This sudden drop in shear force meansthat grease is considered a plastic fluid, and the reduction of shearforce with time makes it thixotropic. Grease is typically manufacturedby first mixing together a mineral oil base stock, a fatty acid or fattyacid ester and an alkali metal salt such as lithium hydroxide. The soapbase stock usually contains about 50% of the final oil content of thegrease.

The low-melting metal nanoparticles of the oil dispersion of the presentinvention will range in size from about 2 nm to about 200 nm, preferablyfrom about 2 nm to about 100 nm, and more preferably from about 2 nm toabout 60 nm. The thickness of the coatings formed will also be in thenano-size range. The coatings of the present invention are substantiallysuperior to conventional coatings intended to reduce the coefficient offriction on interacting surfaces of machinery. For example, aspreviously mentioned, conventional coatings typically have a micron sizegrain structure whereas the coatings of the present invention have ananosize grain structure. This nanosize grain structure results instronger and harder coatings that have superior wear properties comparedto conventional micron size grain structure coatings. The coatings ofthe present invention, because they are substantially thinner thanconventional low coefficient of friction coatings, help maintain thedesigned low clearances between interacting surfaces of machinery.Another advantage of the process of the instant invention is thatconventional processes for applying low coefficient of friction coatingsrequire that the interacting surfaces of a particular piece of machinerybe treated with the low coefficient of friction coating prior toassembly of the machinery. In contrast, practice of one preferredembodiment of the present invention can treat the same interactingsurfaces with a substantially thinner and harder and more wear resistantcoating after the equipment has already been assembled and during itsnormal operating conditions. This can simply be done by replacement ofthe intended conventional lubricating oil with the novel nanoparticlelubricating oil dispersion of this invention. The nanoparticledispersion of the present invention can be replaced periodically as withconventional lubricating oils. Also, after the removal of the novelnanoparticle oil dispersion from the machinery, a conventionallubricating oil, without the novel nanoparticle additives of the presentinvention, can be used in the treated machinery and normal operation cancontinue with reduced friction and wear between the interacting partsbecause the interacting parts will now have a long lasting coating ofnanoparticles.

Other methods, such as dry collection on the vacuum chamber walls or onfilters, can be utilized, but this often causes undesirableagglomeration of the nanoparticles. If this happens, it is difficult tobreak down these agglomerates into smaller more desirable nanoparticleswith current methods, such as media milling Wet collection in lowvolatility lubricating oils not only provides a liquid/solidsdispersion, but it also quenches the molten nanoparticles in their solidstate and preserves the desired nanoparticle size distribution beforethey are able to form larger particles. It also prevents undesiredoxidation of the reactive metal nanoparticles. Although other liquidcollection methods, such as sparging the nanoparticle gas stream throughthe low volatility lubricating oil, or contacting with a film of oil,can be used to form a nanoparticle in oil dispersion, spray collectionis preferred. This is because spray collection provides a more intimatecontact between the lubricating oil and the hot nanoparticles andnanodroplets of molten metal. Without the oil spray cooling process, itis difficult to form a stable nanoparticle particle size distributioncontaining smaller nanoparticles with low melting points (540° C. andbelow).

The heating source for the melting and vaporization of the bismuth, orother suitable metal, can be any source that is capable of providing arelatively constant temperature between about 900° C. and 1800° C.,preferably between about 1200° C. and 1600° C. Non-limiting examples ofheating sources that can be used in the practice of the presentinvention include filament heating and other associated methods, andinduction heating. Induction heating is preferred, particularly using apressed graphite crucible to melt and evaporate the metal. It is alsopreferred that “bumping” of the melted liquid metal be prevented whilethe metal is heated in the evaporation crucible. Bumping can lead to theformation of undesirable large micron-size metal droplets. One preferredmethod to prevent “bumping” is to place a piece of refractory materialin the crucible with the melted metal to mitigate and preferablyeliminate bumping. The refractory material must be one that will notundergo any chemical or physical changes at the temperatures employed.It is preferred that the refractory material be porous, such as a pieceof porous carbon foam. As the metal vapor rises from the crucible duringheating, it comes into contact with the inert gas which provides backpressure in the system that results in the formation of nano-dropletsand nanoparticles from condensation of the molten metal vapor. The inertgas stream, containing metal nano-droplets and nanoparticles, is passedthrough a stream of atomized low volatility lubricating oil and comesinto intimate contact with the newly formed bismuth nanodroplets andnanoparticles. The vapor pressure of the lubricating oil must be lowenough so that an undesirable amount does not vaporize in the system andraise the background pressure in the system beyond the capacity of thevacuum pumps. The oil can be heated to insure proper atomization withinthe system. The oil can also contain one or more non-aqueous stabilizingagents, such as lecithin, in addition to other compounds typically foundin lubricating oils to prevent agglomeration, such as magnesiumsulfonate, wear, such as tricresyl phosphonate; and oxidation, suchamines and phenols. Other lubricant properties, such as pour point andviscosity, can be modified with the addition of polyalkylmethacrylatesand polyolefins, respectively.

After formation, the bismuth nanoparticle/oil dispersion can be filteredthrough a 200 mesh or greater wire filter to separate out the largerparticles and agglomerates that may have formed from molten liquidbuildup on the walls and piping of the system. At this point, the oildispersion can be utilized as a low viscosity lubricant whoseperformance is enhanced over that of conventional lubricants for theintended machinery, even after an initial run time in the machinery andafter the formation of the a low friction nano-coating on interactingparts. However, it is preferred to remove the bismuth nanoparticle oildispersion after an initial run time and after the formation of a lowfriction nano-coatings has formed on the interacting surfaces. Thecoefficient of friction for the nanoparticle oil dispersion is lowerthan that of virgin lubricating oil typically used for the system, butcan contain large abrasive nanoparticles. Thus, is is beneficial toremove the bismuth nanoparticle oil dispersion from the system andreplace it with virgin lubricant. The bismuth oil dispersion can also beutilized as a component in various grease formulations.

The bismuth nanoparticle particle size distribution can be tailored to aspecific operating temperature range of the intendedequipment/machinery. For example, every particle size distribution willhave an optimum temperature at which a low coefficient of frictionnano-coating can be formed on interacting parts. The nano-coatingformation occurs by the melting and sintering of the smaller bismuthnanoparticles in the typical bell-shaped particle size distributioncurve Nanoparticles smaller than the mean diameter in the particle sizedistribution will sinter and melt while the larger nanoparticles willremain substantially solid. At effective concentrations, of bismuthnanoparticles in the lubricating oil (typically below 2 wt. %), lowcoefficient of friction coatings are formed on the interacting surfaces.The concentration of nanoparticles dispersed in the lubricant will rangefrom about 0.001 wt. % to about 2 wt. % based on the total weight oflubricant plus nanoparticles. The entire interacting surfaces will notneed to be covered with the coating of the present invention as long asan effective coating is formed on the interacting surfaces to provide aneffective decrease in coefficient of friction. It preferred thatsubstantially the entire targeted surface of the part be coated. Thatis, at least about 98%, more preferably at least about 99% and mostpreferably 100%. An example of nano-coatings formed by practice of theinvention is shown in FIG. 2 hereof. The coating, as illustrated in thisFIG. 2 hereof resembles a “cobblestone” structure where the larger,unmelted nanoparticles are dispersed in the smaller, meltednanoparticles. The melted nanoparticles act as a binder both to theinteracting surfaces and for the larger, solid nanoparticles of thenano-coating. The coating illustrated in FIG. 2 hereof was obtained withbismuth nanoparticles having an average size of about 50 to 60 nm in aturbine oil at 75° C. The binding action to the contacting surface caninclude alloying to the metal of the surface.

The formation of the low coefficient of friction nano-coating of thepresent invention is dependent on such things as the operatingtemperature of the equipment or machinery containing the nanofluid andconcentration of bismuth nanoparticles in the nanofluid. The term“nanofluid” is introduced herein to mean the nanoparticle/lubricantdispersion used to coat interacting parts. Below the crucial temperaturefor a substantially constant temperature where the smaller nanoparticlesin the particle size distribution start to sinter and begin to adhere tothe interacting surfaces and to other nanoparticles, the nanoparticleswill simply roll between the interacting surfaces and act as ballbearings between the surfaces. This will have friction reducing effecton the friction between the interacting surfaces. As the temperatureincreases, a temperature is reached where the smaller nanoparticles meltand sinter and act as a binding agent between the larger, umeltednanoparticles and the contacting surfaces, forming the nanocoating. Asthe temperature increases further, the ability of the bismuthnanoparticles to form a low coefficient of friction nano-coatings iscompromised and the action of the interacting surfaces results in theformation of larger particle agglomerates of the nanoparticles insteadof pressing the melted/unmelted nanoparticles onto the interactingsurfaces and forming a nano-coating. To compensate for this effect, adecrease in the concentration bismuth nanoparticles in the lubricatingoil, as the temperature increases, allows for the formation of anano-coating having substantially the same particle size distribution;however, the nano-coatings formed will not be as continuous as at thelower temperature and may not result in the desired decrease in thecoefficient of friction or in stability of the coating. Successivetreatments with nanoparticle/lubricating oil dispersion of substantiallythe same concentration will further decrease the coefficient offriction. Eventually, a coefficient of friction will be obtained whichis near, or identical to, that of a pure bismuth coating on theinteracting surfaces. The concentration of the bismuth nanoparticles inthe lubricating oil must be sufficient to contact each of theinteracting surfaces and allow the nanocoatings to form.

The Table below shows the relationship of operating temperature of themachine having interacting surfaces treated in accordance with thepresent invention vesus mean paricle size of the nanoparticles. However,due to the dependence of the nano-coating formation on both theconcentration of the bismuth nanoparticles and the particle sizedistribution, there is overlap of the various operating ranges. Thisindicates the mean particle size range preferred at various temperaturesof the operating machinery, such as gearboxes, transmissions, engines,etc, can be altered by adjustment of the bismuth nanoparticleconcentration. Conversely, the nano-coating formation can occur by theaddition of the lower melting particle size distribution to a highermelting particle size distribution to form thicker nano-coatings ofbismuth material at temperatures where the higher melting pointdistribution would not form an effective nanocoating.

Relationship of Temperature and Particle Size Mean Particle SizeOperating Temperature 2 to 30 nm −40° C. to 60° C.   30 nm to 100 nm 60° C. to 120° C. 100 nm to 200 nm 120° C. to 200° C.

The following examples are presented for illustrative purposes only andare not to be taken as limiting the present invention in any way.

The process of the present invention generally involves the evaporationof bismuth metal in an inert gas condensation process under a vacuum.FIG. 1 hereof is simplified flow diagram of one preferred embodiment ofthe process of the present invention and the method used to obtain thebismuth nanoparticle in lubricating dispersion used in the followingexamples. This figure shows a heating zone H, an oil contacting zone OC,and a collecting zone CZ. All three zones are under vacuum by use ofvacuum pump VP A sample of bismuth to be melted and evaporated is placedin a crucible (not shown) in heating zone H and heated to a temperaturebetween about 800° C. to about 1800° C., preferably to a temperature ofabout 1200° C. to about 1600° C. At these temperatures the bismuth, orother low melting metal, will melt and evaporate. It is preferred thatonly the metal sample and crucible be located in the heating zonebecause of the high temperatures employed. Any ancillary induction coilsand piping will be located outside of the heating chamber. The additionof an inert gas at heating zone H allows for the formation of thenanoparticles. Vacuum pumps VP will keep the system pressure ateffective levels for the formation of various nanoparticle sizes. Inorder to assure that a desired low nanoparticle size distribution beobtained, it is preferred that turbulent flow of inert gas be avoided.Turbulent and high velocity gas flows will have a tendency to destroythe intended particle size distribution of the newly formed bismuthnanoparticles owing to the low melting and sintering temperature of thebismuth nanoparticles. It is believed that this is due to increasedcollisions at turbulent flow between the newly formed nanoparticleswhich leads to undesirable agglomeration and aggregate formation. At thehigh operating temperatures of the process of the present invention thenanoparticles can fuse owing to the low melting temperature of bismuth(274° C.). In addition, nanoparticles contacting each other can alsofuse at temperature below their melting point due to their high surfacearea. This can prevent the formation of the desired and targetedparticle size distribution. The newly formed nanoparticles and inert gasare conducted into spray chamber oil contacting zone OC wherein they arecontacted with a lubricant in the form of a mist or an atomized sprayfrom oil source O. It is preferred that the lubricant or oil be sprayedinto the spay chamber so that there is more intimate contacting of thenewly formed nanoparticles with the lubricant. The nanoparticles arepreferably immediately captured within the spray of lubricant in orderto preserve the desired bismuth nanoparticle size distribution. Theresulting bismuth nanoparticle in lubricating oil dispersion is thenconducted into collection vessel CZ. The concentration of nanoparticlesin lubricating oil will typically be less than 1 wt. %, more typicallyless than 0.5 wt. %. Additional lubrication oil can be used as a diluentto obtain a specific concentration.

EXAMPLE 1

A mean particle size of about 50 to 60 nm is selected for use at 75° C.An initial concentration of about 0.12 wt % with 100 ml of dispersionwas selected for use for reducing friction between two thrust washers ona Falex multi-specimen tester with an 88 pound load and a rotation speedof 600 RPM. A heating mantle on the test fixture was used to adjust thetemperature to 75° C. Coefficient of friction measurements were taken ofa period of about 60 minutes. When 100 ml of the 0.12 wt % bismuthnanoparticle oil dispersion is placed between the two thrust washers andthe test load under rotation is applied, the coefficient of frictiondrops by 50% as compared to the original lubricant oil (Aeroshell 555 )at the same test parameters as is shown in FIG. 3 hereof. Trace A is atrace for original lubricant Aeroshell 555 alone and trace B is for a0.12 wt. % of bismuth nanoparticles dispersed in Aeroshell 555.

Replacement of the bismuth nanofluid between the two thrust washersafter the testing shown in FIG. 3 with fresh original lubricating oil(Aero shell 555) reduces the coefficient of friction by a furtheradditional 25% as compared to that obtained when utilizing the 0.12 wt %bismuth nanoparticle oil dispersion when tested at the identicalparameters of load, RPM and temperature. This is shown in FIG. 4. TraceB represents the use of the fresh lubricant Aeroshell 555 alone andtrace A represents the use of the lubricant Aeroshell 555 containingabout 0.12 wt. % bismuth nanoparticles (identical to the time trace inFIG. 3 for the nanofluid). This further reduction in coefficient offriction indicates the formation of a low friction nanocoating on thecontact surfaces. When examined by scanning electron microscopy (SEM),the “cobblestone” nanocoating in FIG. 2 hereof was observed on thecontact areas of the thrust washers.

EXAMPLE 2

When the temperature of the heating mantle of the test fixture wasraised to 90° C. utilizing the same RPM and load, nanocoating formationand reduction of the coefficient of friction did not occur at the sameconcentration. However, reduction of the bismuth nanoparticleconcentration in the oil does allow the nanocoating formation to occur.For 90° C. with a load of 88 pounds and rotation speed of 600 RPM, thetime traces of a 0.06 wt % bismuth nanoparticle oil dispersion is shownin FIG. 5 hereof with the time traces of the same fixture at the sameconditions with the plain Aeroshell 555. Trace A is the Aeroshell 555oil and Trace B is the 0.06 wt % bismuth nanoparticle oil dispersion. Asteady decrease of the coefficient of friction is observed.

EXAMPLE 3

When the temperature of the heating mantle of the test fixture islowered to 45° C. and the other test parameters of load and RPM kept at88 lbs and 600 RPM, the 60 nm mean particle size no longer forms ananocoating on the contact surfaces of the thrust washers. However, a 30nm mean particle size will lower the coefficient of friction as shown inFIG. 6 hereof. Trace A is the Aeroshell 555 at the identical conditionsas the 0.05 wt % 30 nm bismuth nanoparticle oil dispersion shown inTrace B.

What is claimed is:
 1. A process for applying a low coefficient offriction coating on interacting parts of a machine designed for beinglubricated with a lubricating oil and having a reservoir for storing andproviding lubricating oil, which process comprises: i) dispersing about0.001 wt. % to about 2 wt. % of nanoparticles of one or more metalshaving a melting point less than about 400° C. with a lubricating oil;ii) introducing said dispersion into the reservoir of a machine; andiii) operating the machine under operating conditions for an effectiveamount of time to form a nano-coating on at least a fraction of theinteracting parts of the machine.
 2. The process of claim 1 wherein themetal is selected from bismuth, cadmium, tin, indium, and lead.
 3. Theprocess of claim 2 wherein the metal is bismuth.
 4. The process of claim5 wherein the mean particle size of the metal nanoparticles is fromabout 2 to 60 nm.
 5. The process of claim 1 wherein the lubricant isselected from lubricating oils and greases.
 6. The process of claim 5wherein the lubricant is a lubricating oil.
 7. The process of claim 6wherein the lubricating oil is a natural lubrication oil.
 8. The processof claim 6 wherein the lubricating oil is a synthetic lubricating oil.9. The process of claim 1 wherein the machine is one that powers atransportation vehicle.
 10. The process of claim 1 wherein the machineis an engine for a military vehicle.
 11. The process of claim 3 whereinthe machine is an engine for a military vehicle.